Such sensor devices comprising a sensor unit which includes a emitter, a receiver and a light conductor unit by which a light beam emitted by the emitter is coupled into the window, coupled out of the window and directed onto the receiver, are chiefly used as rain sensors in motor vehicles for automatically actuating the wipers. The use of classical lenses for influencing the optical path, such as the lenses of the rain sensor disclosed in European patent EP 1 068 112 B1, which are inclined against the windscreen, requires a relatively large packaging space.
As known for instance from international application WO 03/026937 A1, smaller constructions are possible by using holographic sensors. These sensors are based on the principle of light diffraction by means of defractive elements and therefore have the principal disadvantage of a substantially smaller yield of usable light and a higher susceptibility to extraneous light.
To reduce the distance between emitter and/or receiver and the light conductor unit in an optical sensor device, German patent DE 196 08 648 C1 proposes to form the light inlet and outlet surfaces of the light conductor unit as Fresnel lenses. Nevertheless, the packaging space required for this device is very large due to the complex light conductor unit, which necessitates an arrangement of the circuit board carrying emitter and receiver vertical to the windscreen.
It is an object of the invention to provide an optical sensor device which is able to scan a very large sensor area, yet only requiring a very small packaging space.